1. Field of the Invention
This invention relates to the manufacture of carbon fibers and activated carbon fibers and more particularly, to an apparatus and method for carbonizing and activating fiber materials.
2. Description of the Prior Art
As is well known in the art, for the manufacture of activated carbon powder or fibers by carbonizing and activating starting organic powders such as coconut shell flour, sawdust and the like, or fibrous materials, it is usual to place the starting powders or materials in an atmosphere of an inert gas at elevated temperatures of 800.degree. to 1200.degree. C. For the activation, a given concentration of an activating gas, such as steam or other oxidative gases, is generally introduced into the inert gas.
In practice, high temperature furnace apparatus have been used for carbonization and activation of such starting materials. The apparatus has an electric furnace using a resistance heater made, for example, of nichrome, MoSi, SiC or the like, in which a ceramic reactor tube is placed. A material to be carbonized and activated is charged into the reactor tube. The furnace is so controlled as to maintain a desired furnace temperature. An inert gas from a carrier gas bomb is passed, for example, through a steam generator into the ceramic reactor tube. An exhaust gas is discharged to outside after treatment with a suitable exhaust gas treating apparatus. In some furnaces, the ceramic reactor tube is rotated to facilitate uniform carbonization and activation.
In the carbonization and activation furnaces, a material to be treated fundamentally undergoes two reactions including a carbonization reaction in an inert gas at high temperatures and a subsequent activation reaction in which the material is reacted with an activating gas at high temperatures. However, existing furnaces are disadvantageous when used in continuous carbonization and activation of continuous materials such as, for example, woven and non-woven fabrics, felts and paper sheets. This is because the control of the atmosphere in the reactor is difficult, so that the resultant carbon fibers and/or activated carbon fibers do not have uniform properties with regard to the strength and specific surface area. In general, the carbonization reaction has to be carried out uniformly under conditions of an atmosphere containing a low oxygen content and a high temperature. On the other hand, the activation reaction using, for example, steam, is a solid-gas reaction between H.sub.2 O molecules and carbon atoms at high temperatures as shown in the following formula (I). Fine pores are formed after removal of carbon atoms by the reaction, thus causing activated carbon to have a high specific surface area. EQU C.sub.n +H.sub.2 O.fwdarw.C.sub.n-1 +CO+H.sub.2 (I)
Such a solid-gas phase reaction is predominantly controlled by three factors including (1) uniformity in temperature of the reaction system, (2) a high and uniform degree of contact between the solid and the gas, and (3) rapid transfer, to outside, of the materials produced by the activation reaction e.g. CO and H.sub.2 gases in the reaction formula (I). If these factors are appropriately controlled, the activation reaction proceeds smoothly, resulting in activated carbon of a high specific surface area. In other words, proper control of the above factors makes it possible to obtain activated carbon of desired characteristics.
In the known batchwise carbonization and activation furnaces using ceramic reactor tubes, the amount and position of a material to be carbonized and activated are held constant, so that the activating conditions indicated above can be relatively easily controlled. Proper control of a concentration and a flow rate of an activating gas and the manner of charging the gas into the reactor tube results in activated carbon with desired characteristic properties. However, the known furnaces are carried out by a batchwise manner, which places an inevitable limitation on the amount of a material to be activated. If the amount is too large, it becomes difficult to obtain uniformly carbonized and activated carbon products. Moreover, when a continuous fiber or cloth article is placed in the furnace in a manner as folded, the outer surfaces of the folded article are more rapidly activated than the inside of the article in the batch-type furnace. Thus, uniform activation is more difficult than in the case of powders.
To avoid the difficulty, cloth articles are generally suspended from a holder as stretched in the furnace in order to ensure uniform contact of an activating gas with the cloth. However, the cloth article where contacted with the holder undergoes activation in a degree different from the other portion of the article which does not contact the holder, with a lower strength. In Japanese Patent Publication No. 57-10205, for example, there is described a procedure of carbonizing and activating fibers in a batch-type furnace in which the fiber in the furnace is heated in air at a rate of 50.degree. to 200.degree. C./hour from room temperature to 450.degree. C. and is subsequently heated in a non-oxidative atmosphere from 700.degree. to 900.degree. C. at the same rate as indicated above. As will be seen from the above, the carbonization and activation reactions in the batch-type furnace essentially require an accurate control of the heating rate and take a long time before the furnace and the resulting activated carbon product are cooled down to room temperature.